This view may be summed up somewhat as follows:-- Cartilaginous jaws would not fossilize, and the Ostracoderms may have possessed them. They may have degenerated from elasmobranchs just as the cyclostomes are supposed to have degenerated. Seeing that bone succeeds cartilage, the presence of bony shields in Cephalaspis, etc., indicates that their precursors were cartilaginous, presumably elasmobranch fishes. Of these arguments the strongest is based on the supposed bony {344}covering of the Osteostraci, with the consequent supposition that their ancestors possessed a cartilaginous covering. This argument is entirely upset, if, as I have pointed out, the structure of the cephalaspid shield is that of muco-cartilage and not of bone. If these plates are a calcified muco-cartilage, then the whole argument for their ancestry from animals with a cartilaginous skeleton falls to the ground, for muco-cartilage is the precursor not only of bone, but also of cartilage itself. The evidence, then, points strongly in favour of Cope's view that the most primitive fishes were Agnatha, after the fashion of cyclostomes, as is also believed by Smith Woodward, Bashford Dean, and Jaekel. Among living animals, as I have shown, the Limulus is the sole survivor of the palæostracan type, and Ammocoetes alone gives a clue to the nature of the cephalaspid, _i.e._ the osteostracan fish. Older than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is it possible from their structure to obtain any clue as to the actual passage from the palæostracan to the vertebrate? Here again, as in the case of the Osteostraci, a relationship to the elasmobranch has been supposed, for the following reasons:-- The latest discoveries in the Silurian and Devonian deposits have brought to light strange forms such as Thelodus and Drepanaspis, of which the latter from the Devonian must, according to Traquair, be included in the Heterostraci. It possessed, as seen in Fig. 139, large plates, after the fashion of Pteraspis, and also many smaller ones. The former, from the upper Silurian, belongs to the Coelolepidæ, and was covered over with shagreen composed of small scutes, after the fashion of an elasmobranch. Traquair suggests that Thelodus arose from the original elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis occurred, and, with still further fusion, Pteraspis. There are always two ways of looking at a question, and it seems to me possible and more probable to turn the matter round and to argue that the original condition of the surface-covering was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a stage on the way to the formation of an elasmobranch, and not a backward stage from the elasmobranch towards Pteraspis.